Except everyone gets magic gear. If it were exclusive to non-casting classes that would be a different thing entirely.
The difference is that gear does one of a few things:
1. Makes your weapon attacks a lot better
2. makes your spells a wee bit better
3. Lets you do magic stuff
The third is helpful for casters, because it lets the do a bit more magic, but transformational for non-magic-classes because it lets them do any magic at all.
Going form flying three times a day to 4 times a day is nice. Going form 0 to 1 is a massive impact.
(In practice, it doesn't take much in the way of magic items to keep things balanced enough unless the players have a mentality of competing with each other that overshadows the rest of the game)